1. Field of the Invention
The present invention relates to implants; and more particularly to an implant that has flexural characteristics and load bearing qualities similar to a natural bone thereby preventing bone loss surrounding the implant due to lack of bone load bearing requirements.
2. Description of the Prior Art
Implants are generally made from bio-compatible materials such as titanium alloys or cobalt chromium alloys. These metallic implants have very high elastic modulus as compared to bone tissue. As a result, any load that is applied is almost entirely carried by the high elastic modulus material sparing the bone of its load bearing function. This results in bone thinning after the implant has healed. A number of prior art patents disclose attempts to solve this problem.
U.S. Pat. No. 4,743,263 to Petrtyl et al. discloses an adaptable isoelastic hip-endoprosthesis. The adaptable isoelastic hip endoprosthesis comprises a joint piece connected to a shaft of the endoprosthesis that is to be implanted in the femur. The shaft is composed of at least two spirally twisted elastic rods. The elastic rods at their lower end are mutually connected to act as stabilization bodies. The adaptable isoelastic hip-endoprosthesis disclosed by the '263 patent is a twisted spiral rod construction which applies force to the prepared bone cavity providing implant stability. When the scar is healed, the spring-like implant is said to form a unitary elastic object. However, the elastic modulus of the rods is very dissimilar to that of bone and the bone may not fuse to the implant due to the absence of a porous bone ingrowth surface. Application of any compressive force to the implant results in further expansion of the twisted rods resulting in permanent enlargement of the bone cavity, which will dislodge the implant.
U.S. Pat. No. 4,851,008 to Johnson discloses a bone implant prosthesis with a substantially stress-free outer surface. This bone implant prosthesis has an outer stress-free surface and a subsurface carried generally beneath and parallel to the outer surface, for bearing tensile stresses. The prosthesis includes a plurality of slots undercutting the outer surface of the prosthesis thus forming a stress bearing subsurface that frees the outer surface from substantial tensile stresses. The stress-free outer surface is readily adaptable to receive a porous coating that enhances bone ingrowth without decreasing the stress resistance characteristics of the prosthesis. The sub-surface is substantially smooth and free of stress concentration sites. The cross bar portions connecting T-shaped slots defines a stress-bearing, discontinuous, smooth subsurface generally parallel to and spaced beneath the outer surface of the prosthesis. The bone implant prosthesis disclosed by the '008 patent utilizes a row of T shaped slots formed by passing an electrode through the bone implant prosthesis melting a straight line path layer over the outer surface with T slots, a high modulus surface. When the implant is flexed, the distal portion of the implant still carries all of the flexural load and therefore does not prevent bone thinning. The portions of the implant through which the electrodes are not passed provides highly stressed areas and the T shaped portions only occupy a minority portion of the overall implant surface.
U.S. Pat. No. 5,549,702 to Ries et al. discloses a flexible orthopaedic stem apparatus. This orthopaedic prosthesis for implantation in a patient's intramedullary canal includes a rigid proximal portion sized and shaped to fit the intramedullary canal of a patient's bone and a lower elongated stem member with an outer surface and distal end. The lower stem member has a helical groove of varying width and depth on the outer surface and a hollowed tapered central core cooperating to enable flexing of the distal end. The flexible orthopaedic stem apparatus of the '072 disclosure has an implant with its distal portion having a helical groove on its surface of varying width and depth while the interior of the stem has a conical core that increases in diameter as it progresses towards the distal tip. This means that the thickness of the distal end decreases to very small value near the distal tip and may permanently deform the implant within the bone after insertion failing to provide adequate support during implant healing. Besides, the permanently deformed tip of the implant may create enormous pain to the patient. The rigid proximal portion of the implant is not indicated to have a porous bone ingrowth coating.
U.S. Pat. No. 6,093,209 to Sanders discloses a proximally hollow prosthesis. The proximally hollow prosthesis has a cap element and a stem. The prosthesis cap element is adapted to a ball and a stem suitable to be implanted in a reamed femoral medullary canal. The stem of the prosthesis has a proximal region and a distal region and is mateable with the cap element of the prosthesis. The proximal region has a circumferential wall defining a hollow cavity terminating to an open proximal end. The hollow cavity has a predetermined length keeping the cavity entirely within proximal region of the stem sufficient to allow the stem to flex, The distal region of the stem is located distal to the isthmus region of the stem and is made of solid metal or alloys. Any flexure of this implant strictly relies on the small wall thickness at the proximal cavity, but the large diameter at the proximal portion has a high moment of inertia providing very little flexing action since the metal or alloy used has a high elastic modulus.
U.S. Pat. No. 6,245,112 to Doubler et al. discloses a joint prosthesis having variable flexibility. The joint prosthesis has a coil spring at its lower end supported by a control rod, which applies compression to the coil spring. Since the central rod supports all the flexibility of the spring, the flexibility of the entire implant is only dependent on the flexibility of the control rod. Since the control rod is held in place securing the spring, any movement in these attachment means will result in unsupported non recoverable movement of the spring.
U.S. Pat. No. 6,723,129 to Dwyer et al. discloses a self-locking modular prosthesis having taper feature and associated method. The modular femoral prosthesis includes a stem member adapted to be implanted into a medullary canal of a femur. The stem member has a continuously tapered elongated bore and a threaded aperture defined therein. A first end of the elongated bore defines a post-receiving opening. The post-receiving opening is defined in a proximal end surface of the stem member. The elongated bore extends between the post-receiving opening and a threaded aperture. The femoral prosthesis also includes a neck member having a neck body, and a head-receiving support member secured to the neck body so as to extend outwardly there from. A tapered post is also secured to the neck body so as to extend outwardly there from. The tapered post is adapted to be received into the elongated bore of the stem member. The tapered post of the stem member has a proximal end and a distal end, and the tapered post is continuously tapered from said proximal end thereof to said distal end. The distal stem component may be provided in a number of different configurations in order to fit the needs of a given patient's anatomy and provide a variety of fixation options (e.g. textures and geometries) and sizes. Moreover, the distal stem component may also be provided in a bow-shaped configuration if required by a given patient's anatomy. This modular implant is not flexible, but is permanently bent according to the femoral cavity shape of the patient.
U.S. Patent Application Publication No. 2002/0111692 to Ralph et al. discloses an artificial hip having a femoral stem portion, which provides for micromovement. The femoral stem member is for use in an artificial hip implant assembly and comprises a shaft portion having a proximal end and a distal end. The distal end is shaped for insertion into the axial bore of a patient's femor. The proximal end includes a ball-shaped surface for insertion into a receiving cup disposed in an acetabular recess in said patient's pelvis. A helical cut is formed in the proximal shaft portion or distal shaft portion such that the helically cut segment of the shaft portion has an elastic modulus. The elastic modulus is substantially less than that of the remaining portions of the stem and approximates that of the patient's bone. The helical cuts form a restoring force spring-like shock absorbing geometry, which permits the rigid metal material to grossly conform to the elastic modulus of the patient's bone. The helical cut is formed in a segment of the proximal end or distal end of the shaft portion and has a polymer sheath that prevents the patient's bone ingrowth into the helical cuts, thus preventing interference with the geometric modification of the overall modulus of elasticity of the helically cut segment. The polymer sheath prevents bone ingrowth and attachment of the implant to the bone. The shaft is solid and does not have a central constant diameter aperture facilitating the bending of the shaft at the distal end.
U.S. Patent Application Publication No. 2004/0107001 to Cheal et al discloses a joint prostheses and components thereof. The joint prostheses of the '001 patent application provides a modular femoral stem that may be rotated after insertion and its neck length adjusted. The fluting or ridge formation in the mid section of the stem is indicated to allow rotation of the implant during insertion and may provide increased flexibility. It is not clear how a longitudinal flute or slot provides any flexibility to any portion of the stem. The '001 patent disclosure does not provide a stem with a proximal potion having bone ingrowth coating and a distal end having a central cylindrical aperture and a helical slot cut in the outer surface to allow flexing of the stem at its distal end.
U.S. Patent Application Publication No. 2007/0233095 to Schlaepfer discloses a device for dynamic stabilization of bones or bone fragments. The device for dynamic stabilization of bones or bone fragments comprises an anchor member for attachment to vertebrae having an opening configured to receive a longitudinal member and the longitudinal member being viscoelastically deformable and having a predetermined bending resilience. This stabilization structure stabilizes the spinal column of a patient.
U.S. Patent Application Publication No. 2008/0161931 to Perez-Cruet et al. discloses a vertebral disc annular fibrosis tensioning and lengthening device. The vertebral disc annular fibrosis tensioning and lengthening device restores the loss of disc height as a result of disc degeneration and other factors. The vertebral disc annular fibrosis tensioning and lengthening device includes pedicle screws having heads with cup-shaped cavities. The pedicle screws are threaded into the vertebral bodies of adjacent vertebrae through the pedicles so that open parts of the heads of the pedicle screws face each other. A spring is inserted into the cup-shaped cavities in compression so that the spring bias forces the pedicle screws apart, thus increasing the height of the disc space. The disclosed device is not a femoral stem.
Foreign Patent Application No. GB2078523, also U.S. Pat. No. 4,314,381 to Koeneman discloses a hip joint prosthesis. The hip joint prosthesis in which the stem portion of the femoral head prosthesis includes at least one layer of elastomeric material disposed between and attaching to at least two sections of rigid material to reduce necrosis and resorption of the adjacent bone. The hip joint prosthesis of the '523 patent application is a stem with two or four metal stem pieces with an elastomer sandwiched between them. This arrangement is indicated to prevent bone resorption due to the reduction of the stiffness of the implanted stem. Specifically, the stem of the '523 patent application does not have a helical slot formed on the outer surface of the distal portion of the stem providing flexibility only at the distal end. Also, the stem of the '523 application is solid and does not have a central cylindrical aperture.
There remains a need in the art for implants of different geometry made from bio-compatible high elastic modulus alloys that can be inserted into bone cavity that does not result in patient pain after the implant has healed and does not result in bone tissue loss due to excess load bearing capacity of the high modulus sparing the bone tissue from its load carrying functionality.